1. Field of the Invention
Exemplary aspects of the present disclosure generally relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photosensitive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may employ a fixing member inside which a heater is provided and a pressing roller pressed against the fixing member to form a fixing nip therebetween. As a recording medium bearing a toner image passes through the fixing nip, the fixing member heated by the heater and the pressing roller apply heat and pressure to the recording medium to melt and soften toner in the toner image, thereby fixing the toner image to the recording medium.
When heating the fixing member at a predetermined temperature by a heat source, if the desired temperature of the fixing member is achieved in a short period of time, a preheating process in a standby state can be shortened or even omitted, thereby allowing significant reduction in consumption of energy. In view of the above, the fixing member employs parts having a low heat capacity such as a thin roller and a thin belt formed of a metal base member on which an elastic rubber layer is disposed. Furthermore, in order to heat the fixing member quickly, a heat source such as a ceramic heater, a halogen heater that heats the fixing member using radiant heat, and an IH (induction heating) type heater with high heating efficiency is used.
In such a known fixing device using the heat sources described above and in a known fixing device in which a fixing member is heated locally such as in a localized-heating type fixing device, an area (heated area) heated by the heat source is located at a place other than the fixing nip portion at which the fixing member and an opposing member such as a pressing roller meet and press against each other, and the unfixed toner image is fixed onto the recording medium under heat and pressure applied in the fixing nip. As a result, even when the fixing member is heated relatively high at the heated area, heat of the fixing device is taken away by the recording medium as the recording medium passes through the fixing nip during fixing operations.
This prevents excessive temperature rise of the fixing member. However, when rotation of the fixing member stops such as when the image forming apparatus is in an OFF-state or the fixing device is in an OFF-mode/sleep mode or energy-saving mode, or during abnormal operations such as paper jams and abrupt halting of the image forming apparatus, residual heat of the heat source keeps heating the fixing member even when the heat source is turned off. Even when the residual heat is not significant, residual heat of a reflector, a stay, and high-temperature internal air heats the fixing member after its rotation.
Furthermore, in a case in which the heated area of the fixing member is spaced apart a certain distance from the fixing nip portion, the heated portion of the fixing member is heated relatively high so as to ensure enough heat for fixing operations until the heated portion of the fixing member arrives at the fixing nip portion. Although advantageous, if the temperature of the fixing member remains high, the fixing member gets damaged. In particular, if the fixing member is formed of parts having a low heat capacity, such as a thin roller and a thin belt to shorten the warm-up time and reduce power consumption, the fixing member is damaged easily.
When the image forming apparatus equipped with the fixing device of the localized-heating type, the pressing member and a recording medium contacting the fixing member draw heat from the fixing member. However, other areas of the fixing member not contacting the pressing member and the recording medium, in particular, the area including the heated portion before entering the fixing nip portion, reserve heat due to a relatively low heat transfer ratio in the circumferential direction. As a result, deformation or thermal expansion occurs at the area.
A relatively large temperature deviation in the circumferential direction of the fixing member causes a difference in the degree of thermal expansion at a high-temperature area and at a low-temperature area. If the difference between the high-temperature area and the low-temperature area is significant, deformation also known as a kink which is a depression formed in the fixing member occurs in the center of the high-temperature area of the fixing member. Although the deformation is caused by local thermal expansion, the degree of which depends on the material and the thickness of the fixing member, such deformation in the fixing member causes an image defect as well as damage to the fixing member.
In view of the above, there is demand for a fixing device capable of preventing deformation of a fixing member even when the fixing member stops abruptly and the temperature thereof increases excessively.